I post this as I am about to encounter a similar situation. J
Exchange MVP Henrik Walther has released a new article called Outlook Web Access 2003 Forms-based Authentication and the default domain dilemma.
Okay so you have enabled Forms-based Authentication (FBA) and your users can now logon to Outlook Web Access (OWA) 2003 either by typing domain\username or email@example.com. But ever since you enabled FBA your users have found it annoying they can’t type username anymore, the other day even the CEO told you his opinion about it, and he definitely wasn’t satisfied so you better come up with a solution.
[MS Exchange Blog]